Overburden strata failure due to mining

China has developed some specific methods of coal mining and experimental techniques under aquifers and surface water. Over the last 40 years, about 1000 longwall faces were extracted under surface and ground water, liberating millions of tons of coal reserves without disastrous consequences. Since coal extraction enhances hydraulic conductivity, it is desirable to determine accurately pre- and post-mining hydraulic conductivities in the overburden strata. To measure these conductivities, boreholes are drilled pre- and post-mining either on the surface or in underground observing roadways. The flow rate or circulation loss along the borehole during drilling is measured by pumping drilling mud into the borehole. Well logs are also applicable for the determination of mining induced fractures and permeability changes (Peng et al. 2002a).

In general, two failure zones that affect strata hydraulic conductivity are formed overlying the mined area: a caved zone and a water-conducting fractured zone (Liu et al. 1981, Zhang and Shen 2004). For mining under aquifers, the water-conducting fractured zone is more interesting, since it provides access for water inflow into the mine workings because of hydraulic conductivity enhancement in this zone. From in-situ testing of borehole flow rate, the water-conducting fractured zone can be divided into the following three subzones (Fig. 9.1):

(1) Slightly fractured subzone. Only little fractures are induced in the strata. Compared to the original strata, hydraulic conductivity in this zone increases slightly. The fluid circulation loss rates in the observing borehole are less than 0.1 l/s m;

(2) Moderately fractured subzone. The strata only have partial bed separations and fractures. Hydraulic conductivity in the strata increases moderately. The circulation loss rates are between 0.1 and 1.0 l/s m;

(3) Severely fractured subzone. Most of the strata have been fractured, and the fractures are interconnected. Hydraulic conductivity in the strata increases dramatically. The circulation loss rates are greater than 1.0 l/s m.

Field observations by circulation loss measurements in boreholes while drilling have shown that the strata failure characteristics differ considerably for different inclinations of the extracted seams. For flat or slightly inclined coal seams (the dip angle, D < 30q), the profile of the water-conducting fractured zone is broad in section with extended lobes over the headgate and tailgate, as shown in Fig. 9.2. For strong rocks, the failure zone has a different characteristic, as shown in Fig. 9.3, which is that the failure zones are much higher in the vertical direction and narrower in section.